Hitherto, focus detection devices have used contrast detection means (or a beam splitter), which uses light receiving elements arranged in two arrays such that two light beams obtained as a result of splitting a light beam are made to be incident on corresponding light receiving elements in the respective light receiving element arrays, and there has been an amplitude division method which is used for such detecting means and which makes use of a semi-transparent mirror. The ratio between the transmittivity and reflectivity of such a semi-transparent mirror is varied by depositing a metal (such as Al, Ag or Au) or a non-metal (such as ZnS, CeO or TiO.sub.2) to a suitable thickness.
However, it has been very difficult to accurately control the ratio of the light beam splitting with the film thickness. In addition, light is greatly absorbed, and also the division ratio is subject to great variations with respect to polarization and wavelengths.
Meanwhile, there has also been in practice a method which is based not on amplitude splitting, such as with the use of a semi-transparent mirror, but on wavefront splitting in a split image system for a focus detecting device. In the split image system, light receiving elements are used which are arranged in a row such that two adjacent light receiving elements cooperate with each other to receive respective split light beams. However, no system which uses two light receiving element arrays having corresponding light receiving elements spaced apart from one another to form respective light receiving element pairs to receive respective split light beams, has been used in any split image system or contrast system involving wavefront splitting of a light beam.